Airborne particles play critical roles in air quality, health effects, visibility, and climate. Secondary organic aerosols (SOA) formed from oxidation of organic gases such as α-pinene account for a significant portion of total airborne particle mass. Current atmospheric models typically incorporate the assumption that SOA mass is a liquid into which semivolatile organic compounds undergo instantaneous equilibrium partitioning to grow the particles into the size range important for light scattering and cloud condensation nuclei activity. We report studies of particles from the oxidation of α-pinene by ozone and NO 3 radicals at room temperature. SOA is primarily formed from low-volatility ozonolysis products, with a small contribution from higher volatility organic nitrates from the NO 3 reaction. Contrary to expectations, the particulate nitrate concentration is not consistent with equilibrium partitioning between the gas phase and a liquid particle. Rather the fraction of organic nitrates in the particles is only explained by irreversible, kinetically determined uptake of the nitrates on existing particles, with an uptake coefficient that is 1.6% of that for the ozonolysis products. If the nonequilibrium particle formation and growth observed in this atmospherically important system is a general phenomenon in the atmosphere, aerosol models may need to be reformulated. The reformulation of aerosol models could impact the predicted evolution of SOA in the atmosphere both outdoors and indoors, its role in heterogeneous chemistry, its projected impacts on air quality, visibility, and climate, and hence the development of reliable control strategies.atmospheric aerosol | nitrate radical | kinetic growth mechanism | condensation mechanism A irborne particles are well-known to negatively affect human health (1) and to contribute to "haze" associated with urban and regional pollution, leading to a reduction in visibility (2). On a global scale, airborne particles scatter solar radiation and can act as cloud condensation (CCN) and ice nuclei (IN), influencing the radiative balance of the atmosphere (3, 4). Currently these effects represent the largest uncertainty in calculations of climate change (5). A major component of atmospheric particles is secondary organic aerosol (SOA) formed via the oxidation of gaseous anthropogenic and biogenic precursor compounds. The SOA material is formed from low-volatility oxidation products (3, 4). However, the processes and species leading to SOA formation and growth are not fully understood, which precludes reliable quantitative predictions of their impacts on climate, visibility, and human health.Regional and global chemical models have generally underpredicted SOA concentrations compared to those from field measurements (6-9). Inclusion of a number of additional factors such as new SOA precursors, condensed phase chemistry, updated gasphase chemistry and SOA yields, new primary semivolatile and intermediate volatility species, and improved emissions inventories of both gases and p...

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Gaseous HCl generated from a variety of sources is ubiquitous in both outdoor and indoor air. Oxides of nitrogen (NO y ) are also globally distributed, because NO formed in combustion processes is oxidized to NO 2 , HNO 3 , N 2 O 5 and a variety of other nitrogen oxides during transport. Deposition of HCl and NO y onto surfaces is commonly regarded as providing permanent removal mechanisms. However, we show here a new surface-mediated coupling of nitrogen oxide and halogen activation cycles in which uptake of gaseous NO 2 or N 2 O 5 on solid substrates generates adsorbed intermediates that react with HCl to generate gaseous nitrosyl chloride (ClNO) and nitryl chloride (ClNO 2 ), respectively. These are potentially harmful gases that photolyze to form highly reactive chlorine atoms. The reactions are shown both experimentally and theoretically to be enhanced by water, a surprising result given the availability of competing hydrolysis reaction pathways. Airshed modeling incorporating HCl generated from sea salt shows that in coastal urban regions, this heterogeneous chemistry increases surface-level ozone, a criteria air pollutant, greenhouse gas and source of atmospheric oxidants. In addition, it may contribute to recently measured high levels of ClNO 2 in the polluted coastal marine boundary layer. This work also suggests the potential for chlorine atom chemistry to occur indoors where significant concentrations of oxides of nitrogen and HCl coexist.